Conventionally, the timeslot method has been used for a communication between a contactless card that sends/receives data and a reader/writer that identifies the contactless card, using the electromagnetic induction method or the like. In the case where plural contactless cards simultaneously exist within a communication area covered by a reader/writer and such plurality of contactless cards simultaneously send response signals in response to polling performed by the reader/writer, any of the contactless cards cannot normally communicate with the reader/writer due to the collision of response signals. The timeslot method prevents the occurrence of such situations.
The following describes how a communication is carried out in accordance with the timeslot method.
(1) The reader/writer sends a request command in order to detect the presence of a contactless card. The request command includes “slot number” which is the number of time periods (timeslots) in which the contactless card can make an initial response.
(2) After receiving the request command, the contactless card returns an initial response that includes its card ID in one of the timeslots. The contactless card determines, on its own, a timeslot in which it makes an initial response.
(3) When detecting a collision of initial responses, the reader/writer sends a request command again. As a result, the counting of the timeslots restarts.
(4) The reader/writer can identify all contactless cards when no collision of initial responses is detected in any of the timeslots. This marks the completion of the card identification processing.
As a contactless card identification method employed by a reader/writer, there also exists the slot maker method in which a reader/writer notifies a contactless card of timing at which the contactless card can make an initial response. Also in this method, the contactless card determines, on its own, a timeslot in which it makes an initial response.
In order that a reader/writer identifies each contactless card, a contactless card is required to use a card ID by which it is possible to distinguish such contactless card from another contactless card. Through the use of a card ID, the reader/writer identifies a contactless card such reader/writer is communicating with. The above methods are standardized as ISO/IEC14443-3 (Type B, initialization and anticollision scheme).